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Aucklanders have been warned their coastline harbours the same potentially fatal toxic shellfish that have led to a record spate of poisonings in the Bay of Plenty.

The Auckland Regional Public Health Service yesterday said the risk of serious shellfish poisoning was high in Auckland and Northland.

Health officials have labelled a recent outbreak of paralytic shellfish poisoning the worst recorded in the Bay of Plenty. Twenty people have been hit in the past week alone, with 13 admitted to hospitals so far this month with presumed shellfish poisoning. Most had eaten pipi or tuatua gathered around Papamoa.

Their symptoms ranged from tingling around the mouth to difficulty walking, and two were admitted to intensive care units.

Last night one patient remained in a serious but stable condition in intensive care at Whakatane Hospital, with two more patients in the general ward of Tauranga Hospital.

The offending toxin comes from ocean algae which shellfish eat, concentrating the toxin in their flesh.

There have been no reported poisonings in the Auckland region, but health authorities said "the very same danger" existed.

"This can be a fatal illness. And 20 people have been affected in the Bay of Plenty," said Dr Cathy Pikholz, medical officer of health for the Auckland service. "It's a very serious issue; people should heed the warning as it can kill you."

People were advised not to eat shellfish collected from Mohakatino (in Taranaki) north to Maunganui Bluff (north of Dargaville), including the Manukau and Kaipara Harbours. Shellfish from the Waitemata Harbour and North Shore beaches were safe at this stage.

In affected areas, shellfish including kina, mussels, toheroa, pipi, tuatua, oysters, cockles and scallops should not be eaten. Cooking or freezing does not make them safe, and toxic shellfish do not look or taste different. Paua, crayfish and crabs can be collected but the gut should be removed.